Homoiconicity is orthogonal to local reasoning. It just means the syntax is represented by the native data format, nested lists. This does make code generation very straightforward with list processing primitives.
replyMutation in scheme is possible, via set!, and set-car! and set-cdr!, but it's not recommended. Functional program design side-steps the issue.
see also:
SICP: 3.1.3 The Costs of Introducing Assignment
https://sarabander.github.io/sicp/html/3_002e1.xhtml#g_t3_00...